The leaking windscreen liquid seem to be a fairly common fault, but the dealer is suppose to fix this free of charge during your warranty. Be happy that it did break before you run out of warranty
The engine mount problem seem to be more common in colder climates, and it seem to get worse during the winter.
About the fuel economy, it will improve with time, after I put 4000 miles on the engine I noticed better fuel economy, the engine were also smoother to run on lower revs as well. The engine is pretty horrible when it's new, but believe me it will get better with time, my engine is still improving, will be intresting to see how it's going to run once i hot 10.000miles+.
I started up with the same fuel economy as you (around 40-41mpg), and now i get 44-45 mpg (driving around town subtropical climate with aircon off). Haven't been doing much highway driving yet. If i run the aircon on, the fuel economy is just horrible...
The thing is with this engine, you have to keep your revs low to get a good fuel economy. I usualy keep them between 1.500 - 2.500, sometimes i push it up to 3000rpm if i switch straight from 3rd gear to 5th.